Apparatus and method for treating sesquisulfate waste streams

ABSTRACT

The specification discloses an apparatus and method for treating a slurry containing sodium sesquisulfate to recover sulfate and acid constituents therefrom. The apparatus includes a treatment vessel having a separation wall delineating a clarifying zone and a mixing zone. Slurry containing sesquisulfate crystals is introduced into the mixing zone along with water and the material is mixed to promote dissolution of the crystals and formation of sodium sulfate solids. Sodium sulfate solids are collected in a lower portion of the treatment vessel and conveyed out of the vessel, and liquid from the clarifying zone is conducted from an upper end of the treatment vessel to a conventional liquid processing unit. Treatment of a sesquisulfate-containing slurry in accordance with the invention provides sodium sulfate containing little or no sesquisulfate crystals thereby reducing the need for vacuum filtration or other expensive separation techniques to recover sulfuric acid and sodium sulfate solids from the slurry.

FIELD OF THE INVENTION

This invention relates generally to the treatment of sesquisulfate wastestreams to recover acid and sulfate values therefrom. More particularly,the invention relates to an apparatus and method for treating a sodiumsesquisulfate (Na₃ H(SO₄)₂)-containing waste stream from a chlorinedioxide production process to recover a sulfuric acid (H₂ SO₄) solutionand sodium sulfate solids (Na₂ SO₄).

BACKGROUND OF THE INVENTION

Bleaching or whitening of pulp is typically accomplished by chemicallyaltering and/or removing colored matter in the pulp and imparting ahigher brightness thereto. Chlorine-based chemicals such as chlorine,chlorine dioxide and hypochlorite have been used in pulp bleaching formany years and continue to be used for removing lignin and bleaching thepulp to high brightness. Growing environmental concerns have led to anincrease in the use of chlorine dioxide and nonelemental chlorine agentssuch as oxygen, peroxide and/or ozone in bleaching processes.

Waste streams containing sodium sesquisulfate result from the productionof chlorine dioxide for use in bleaching pulp. The increasing use ofchlorine dioxide in pulp mills thus makes recovery of the acid andsulfate values of such waste streams of considerable economic importancesince the components recoverable from the waste streams may be recycledfor a variety of uses.

One process for recovering sodium sulfate and sulfuric acid from astream containing sodium sesquisulfate is described in U.S. Pat. No.5,116,595 and is commonly referred to as the "R10" process. The '595patent is specifically directed to treatment of a waste stream obtainedfrom a process for generating chlorine dioxide for use in the pulp millcommonly known in the art as the "R8" process, and described in U.S.Pat. No. 4,081,520.

In accordance with the method described in the '595 patent, a slurrycontaining sulfuric acid and sodium sesquisulfate is filtered, contactedwith water, and thereafter filtered again to separate the resultingsodium sulfate precipitate from the sulfuric acid solution. Onesignificant disadvantage of recovery processes of this type is that theyrequire filtration techniques, typically vacuum filtration techniques,and the concomitant equipment and operational costs associated withthese filtration steps. However, such filtration techniques have beenconsidered vital to achieving sulfate solids of satisfactory yield.

Accordingly, it is an object of the present invention to provide amethod and apparatus for treating a sesquisulfate-containing stream torecover valuable constituents thereof.

It is an additional object of the invention to provide an apparatus andmethod for treating a sesquisulfate stream to recover acid and sulfatevalues of the stream.

Another object of the invention is to provide an apparatus and methodfor treating a sodium sesquisulfate-containing waste stream to recover asulfuric acid solution and a sodium sulfate solids.

A further object of the invention is to provide an apparatus and methodof the character described which avoids or limits the need formechanical filtration such as vacuum filtration.

Still another object of the invention is to provide an apparatus andmethod of the character described which is uncomplicated and economicalas compared to conventional methods and which does not compromise thequality or quantity of the yield.

SUMMARY OF THE INVENTION

With regard to the foregoing and other objects, the invention providesan apparatus for treating a slurry containing sodium sesquisulfatecrystals to recover sulfate and acid constituents therefrom. In general,the apparatus includes an upright treatment vessel having an upper endand a lower end and a separation wall defining on one side a mixing zoneand on another side a clarifying zone. The separation wall includes alower edge spaced above the lower end of the vessel to enable fluid flowcommunication between the mixing zone and the clarifying zone by flow ofliquid under the lower edge between the zones.

A collector is located within the treatment vessel generally below themixing zone and includes an upwardly opening reservoir for collectingsolid particles descending by gravity from the mixing zone. A diffuserin the reservoir of the collector is connected in flow communicationwith a source of pressurized gas for releasing a flow of gas bubblesthrough solid particles collected in the reservoir to agitate particlescontained therein, and to provide a flow of gas bubbles up through themixing zone to promote a turbulent mixing flow regime within the mixingzone for dissolution of sodium sesquisulfate crystals and formation ofsodium sulfate solids.

An inlet conduit is connected in flow communication with the mixing zonefor delivering the slurry into the mixing zone. A water inlet isprovided for introducing water into the mixing zone for mixing with theslurry.

An outlet conduit adjacent the upper end of the treatment vessel isprovided for directing liquid from the clarifying zone to a conventionalliquid processing unit. A solids outlet is provided adjacent the lowerend of the treatment vessel for directing sodium sulfate solids out ofthe vessel.

In a preferred embodiment, the separation wall is provided by an uprightelongate cylindrical conduit generally centrally located within thetreatment vessel. The mixing zone is defined within the conduit as anelongate upright cylindrical columnar space and the clarifying zone isdefined outside of the conduit as an elongate upright annular spacebetween the conduit and the inner wall of the vessel. The lower edge ofthe wall is defined as the lower continuous edge of a generally conical,downwardly opening flow expansion member on the conduit.

The invention also provides a method for treating a slurry containingsodium sesquisulfate crystals to recover a sodium sulfate solidstherefrom. In accordance with the method, the slurry is conductedthrough an elongate generally upright mixing zone in a treatment vesseland is mixed therein to promote dissolution of the crystals andformation of sodium sulfate solids. The slurry is then conducted to aclarifying zone.

Sesquisulfate crystals descending from the mixing zone are collected inthe reservoir of a collector located beneath the mixing zone. Gas isbubbled through the crystals in the reservoir to promote dissolutionthereof and to provide a flow of gas bubbles ascending through themixing zone to promote a turbulent flow regime therein and consequentmixing of the slurry. Sodium sulfate solids descending from the slurryin the clarifying zone are collected and conducted out of the vesselalong with entrained liquid consisting principally of sulfuric acid andwater. The balance of the liquid from the clarifying zone is conductedout of the vessel along with a small amount of entrained solidsconsisting principally of sodium sulfate.

Treatment of a sesquisulfate-containing stream in accordance with theinvention yields an effluent containing sulfuric acid, water and a smallamount of dissolved sodium sulfate and a sodium sulfate solids productwith entrained sulfuric acid containing little or no sesquisulfatecrystals, limiting the need for vacuum filtration and other expensive,complicated separation techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and aspects of the present invention willbecome further known from the following detailed description consideredin conjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a preferred embodiment of anapparatus for treating sodium sesquisulfate in accordance with thepresent invention;

FIG. 2 is a perspective view of a preferred embodiment of a mixingconduit and collector for use in practicing the invention; and

FIG. 3 is a perspective view of another embodiment of a mixing conduitand collector for use in practicing the invention.

DETAILED DESCRIPTION

With initial reference to FIG. 1, there is shown a preferred embodimentof a separator 10 in accordance with the invention for continuouslytreating waste streams containing sodium sesquisulfate crystals (Na₃H(SO₄)₂) to recover acid and sulfate constituents thereof, namely, asulfuric acid (H₂ SO₄) solution and sodium sulfate solids (Na₂ SO₄).

In general, a waste stream containing sesquisulfate crystals isintroduced into the separator 10 and treated to yield a sulfuric acidsolution effluent L and sodium sulfate solids S. The effluent L willgenerally contain water, sulfuric acid and a small amount of sodiumsulfate solids. The solids S will generally contain principally sodiumsulfate solids (i.e., crystalline anhydrous neutral sodium sulfate),along with from about 2 to about 10 wt. % sulfuric acid at a normalityof from about 0.2 N to about 1.5 N and it may contain a small amount ofsodium sesquisulfate crystals.

The fluid-contacting surfaces of separator 10 are preferably made ofconventional materials such as stainless steel or a polymer-lined orpolymeric material suitable for the processing materials such as sodiumsesquisulfate and sulfuric acid. As will be explained below, theseparator is configured to provide several zones which cooperate toaccomplish the desired treatment of the sesquisulfate stream.

In a preferred embodiment, separator 10 includes an upright vessel 11having a generally cylindrical upper portion 12, a generally funnel orinverted cone-shaped middle portion 14 and a generally cylindrical lowerneck portion 16. The separator 10 also includes an upright elongategenerally cylindrical conduit 18 defining a mixing zone 19 for treatinga sodium sesquisulfate slurry 20 provided through conduit 22 from awaste source 24 such as a waste stream in a pulp mill. Conduit 18provides a separation wall for separating mixing zone 19 from aclarifying zone 21.

Water 26 for a dilution of the slurry 20 is provided through conduit 28from a water source 30. The slurry conduit 22 and water conduit 28 areprovided with suitable flow control valves and pumps of a type known inthe art for controlling the flow of slurry 20 and water 26 to the mixingzone 19.

Conduit 18 includes an inlet opening 32 adjacent an upper portion 34thereof for receiving the slurry 20 and water 26 into the mixing zone19. The conduit 18 also contains a lower edge 36 spaced above the lowerneck portion 16 of the vessel 11 to enable fluid flow communicationbetween the mixing zone 19 and clarifying zone 21. In the illustratedembodiment, liquid from the mixing zone 19 together with entrainedsolids flows under the lower edge 36 of the conduit 18 into theclarifying zone 21.

Conduit 18 preferably has an elongate central cylindrical portion 40 anda lower conical portion 41, and is generally centrally located withinthe treatment vessel 11. By virtue of this arrangement, clarifying zone21 is defined as a generally annular space between the outer surface ofconduit 18 and the inner surface of the upper portion 12 of vessel 11and, moving downwardly from upper portion 12, converges or is narrowedaccording to the shape of middle position 14 to neck portion 16 so as tofunnel solid particles descending through zone 21 to the bottom-mostportion of vessel 11. Conical portion 41 of conduit 18 expands the flowarea of material moving down through the conduit thereby deceleratingthe material and providing a transition zone 42 before the slurry isreleased into clarifying zone 21.

Conduit 18 may be supported within vessel 11 by any suitable means suchas a plurality of horizontally spaced apart radially extending bars orrods 43 connected in supporting relation between the outer surface ofconduit 18 and the inner surface of the upper portion 12 of vessel 11.

As will be explained more fully below, the sesquisulfate stream ispreferably contacted with water in the mixing zone 19 under conditionswhich promote dissolution of sodium sesquisulfate crystals to sodiumsulfate solids. In this regard, and without being bound by theory, it isbelieved that the sesquisulfate crystals dissolve in the water with thesulfate saltcake forming as a precipitate.

Water 26 is preferably introduced into the zone 19 with thesesquisulfate slurry 20 as previously described and/or water is flowedupwardly into the lower portion 41 of the mixing zone 19 to promotesubstantially turbulent conditions in the mixing zone 19 along withdilution of the slurry.

As used herein, the term "turbulent" is a relative term and will beunderstood to refer generally to non-stagnant fluid conditions so as topromote dissolution of the sesquisulfate crystals. Those of ordinaryskill will recognize that consistent with the aim of the invention topromote dissolution of the sesquisulfate crystals in the slurry fed tothe separator 10, a sufficient turbulence is applied in the mixing zone19 to substantially accomplish this objective. The degree of turbulencewill vary depending on the circumstances of feed concentration,temperature, vessel dimensions and the like and may be determined in theexercise of ordinary skill as appropriate to accomplish the inventionobjectives on a case-by-case basis.

The treated slurry emerging from the mixing zone 19 containing sodiumsulfate solids, sulfuric acid and/or water discharges into the lessturbulent clarifying zone 21 thereby promoting descension of sodiumsulfate solids which settle in the lower neck portion 16 of the vessel11 and accumulate as solids S which are discharged from the vessel 11.The liquid effluent L in clarifying zone 21 is discharged from theseparator 10 through conduit 44 in the upper cylindrical portion 12 ofthe vessel 11. A control valve 46 controls the flow of effluent L fromthe separator 10 so that a substantially constant flow is maintained.The effluent L from the separator 10 is conducted to a conventionaltreatment system 48 for concentration of sulfuric acid, removal of anyentrained solids and recycle to the mill.

Additional water from a source 50 may be introduced into the lowerportion 52 of the vessel 11 via a conduit 54 and associated valve 56 forfurther dilution of slurry 20. A flow distributor 58 is preferablyprovided on the end of the conduit 54 to direct a diffused flow of waterin a generally upwardly and radially outward direction into theclarifying zone 21.

With additional reference to FIG. 2, a generally inverted cone-shapedcollector 60 is preferably provided in the separator 10 positioned belowmixing zone 19 adjacent the middle portion 14 of the separator insubstantial vertical alignment with and spaced below lower conicalportion 41 of the conduit 18. Collector 60 defines an upwardly openingreservoir 61 therein for intercepting and collecting undissolved sodiumsesquisulfate crystals 78 which descend from transition zone 42. Thediameter and configuration of the opening of reservoir 61 defined bycollector 60 preferably corresponds substantially to that of the centralcylindrical portion 40 of conduit 18 to catch a substantial portion ofthe relatively large sodium sesquisulfate crystals 78 descending fromthe conduit.

To further promote mixing and dissolution of sesquisulfate crystalswithin mixing zone 19, a flow of gas such as air from a source ofpressurized gas 62 is preferably emitted from collector 60 at a rate offrom about 10 to about 80 cubic meters per ton of sesquisulfate via agas inlet conduit 64 which doubles as a support for collector 60. Gasflow may be regulated as by flow control valve 66.

Gas inlet conduit 64 is preferably connected to the collector 60 at itsbase 68 in order to introduce a flow of gas bubbles 69 upwardly throughthe reservoir 61. As will be appreciated, a flow of gas bubbles fromcollector 60 moves generally in the direction indicated by arrows C upthrough the mixing zone 19 to promote turbulence which aids indissolution of sesquisulfate crystals therein. In this connection, it isfurther noted that expanded conical portion 41 of conduit 18 serves tocollect gas bubbles 69 ascending from collector 60 into the mixing zone19. Accordingly, the lower edge 36 of conical portion 41 is preferablydimensioned and positioned so that the conical portion 41 in the mannerof a hood substantially envelopes and thus captures bubbles ascendingfrom collector 60, while decelerating the downward flow of material fromcentral portion 40 of the mixing zone 19 and also accelerating the flowof bubbles 69 through the transition area for an enhanced mixing effect.

A screen 70 is preferably provided at the base 68 of the collector 60 inreservoir 61 to diffuse the flow of gas into the small bubbles 69 and tolimit entry of solids into the conduit 64. Conduit 64 also preferablyincludes a U-shaped bend or trap 72 to collect solids which may enterthe conduit 64 and to limit further migration of solids in the conduit64.

Thus, it will be appreciated that in operation sesquisulfate and waterintroduced into upper portion 34 of the conduit 18 flows downwardlythrough mixing zone 19 toward the lower portion 41 countercurrently withthe upward flow of bubbles 69. In this connection, it is noted thatupper portion 34 of conduit 18 is preferably flared outwardly at theinlet opening 32 thereof so as to funnel the entering water and sodiumsesquisulfate slurry toward the central portion 40 of the conduit 18 topromote integration and mixing of the water and sesquisulfate therein.Additional mixing in zone 19 may be provided by flights projecting intothe zone and/or one or more motor-driven impellers.

It has been found that the substantially turbulent mixing ofsesquisulfate stream and water in accordance with the invention promotesa dissolution of the crystals in the water. However, some portion of thesesquisulfate crystals, particularly the larger crystals, may not fullydissolute prior to exiting the mixing zone 19. Accordingly, collector 60is positioned below the lower portion 41 of the conduit 18 to interceptand collect a substantial portion of such undissoluted crystals 78. Itis noted that the larger of these crystals 78 may tend to stay withinthe reservoir 61 of collector 60 and undergo repeated rising andfalling, colliding with other particles, and that as these crystals growsmaller, they may become entrained and carried back into the mixing zone19 by the bubbles 69 eventually dissoluting and settling as sodiumsulfate solids S.

Material exiting the mixing zone 19 (excepting solids setting intocollector 60) enters the clarifying zone 21 as indicated by arrows R. Asthe material flows out of the mixing zone 19 through transition zone 42it expands radially and its velocity decreases. Sodium sulfate solidsdescend within the relative calm of the clarifying zone 21 under theinfluence of gravity and collect as solids S in the lower neck portion16 of vessel 11. Substantially solids-free liquid effluent rises throughthe clarifying zone 21 from the lower edge 36 of the conduit 18 and isdirected out of vessel in conduit 44. Thus, an increasing solidsgradient will generally be observed moving downwardly through clarifyingzone 21.

Solids S may be removed from separator 10 via a solids effluent outletdelivery conduit 80 and associated control valve 82 located adjacent thelower neck portion 16 of the separator 10 and thereafter furtherprocessed using conventional recovery techniques in a recovery unitindicated generally at 84 to recover sodium sulfate substantially freeof entrained sulfuric acid. Outlet delivery conduit 80 may be anyconventional wet solids conveyance apparatus such as a screw conveyor orthe like.

It is further noted that separator 10 may be provided in variousgeometrical configurations without deviating from the spirit of theinvention. For example, FIG. 3 shows an alternate configuration forconduit 18, designated 18', and for the collector 60, designated 60'. Ascan be seen, conduit 18' includes upper and lower portions 34' and 41'provided by spaced-apart elongate panel members 90 which angle outwardlyfrom a pair of uniformly spaced apart upright panel members 86 and 88which define between them mixing zone 19'. Likewise, the collector 60'is provided by a pair of side panel members 92 and 94 which angleoutwardly from a slotted tube or screen 70' in flow communication with aconduit 64' for delivering air bubbles from the collector 60' up intothe mixing zone 19' in a manner substantially equivalent to thatdescribed previously.

With respect to the operation of the separator 10 according to theinvention, the sodium sesquisulfate slurry treated in the separator 10may be a waste stream from a pulp mill which contains sodiumsesquisulfate crystals, e.g., such as may be produced during thegeneration of chlorine dioxide for use in the mill. Such waste streamstypically contain from about 30 to about 80 percent by weight solids,most typically about 80 weight percent solids, which solids consistprimarily of sodium sesquisulfate crystals and sodium sulfate solids. Byvolume, the slurry typically contains from about 2 to 3 parts sodiumsesquisulfate crystals per 1 part sulfuric acid and such feed typicallyhas an acid normality of from about 0.5 to about 4.8 N.

The sesquisulfate slurry introduced into the mixing zone 19 via theconduit 20 preferably has a temperature of from about 20° C. to about60° C. and water introduced into the mixing zone 19 via conduits 28 and54 preferably has a temperature of from about 20° C. to about 100° C. Ifnecessary, the incoming streams may be passed through one or more heatexchangers to obtain the desired inlet temperatures.

In an exemplary embodiment water is conducted to the mixing zone 19 viathe conduit 28 at a rate of from about 0 liter/Kg sesquisulfate to about0.5 liter/Kg sesquisulfate and via the conduit 54 at a rate of fromabout 0.1 liter/Kg sesquisulfate to about 1 liter/Kg sesquisulfate. Itis preferred that the amount of water and sesquisulfate introduced becontrolled such that the ratio of the total weight of water to the totalweight of sesquisulfate is from about 0.6 to about 1.0, preferably fromabout 0.7 to about 0.9 and the slurry diluted by a factor of from about0.2 to about 0.5.

As will be appreciated, the invention offers economic advantages overconventional processes and apparatus which involve the use of expensivevacuum filtration equipment. By avoiding or limiting the use of suchequipment, considerable savings may be achieved without sacrificing thequality or quantity of the yield.

The foregoing description of certain embodiments of the presentinvention has been provided for purposes of illustration only, and it isunderstood that numerous modifications or alterations may be madewithout departing from the spirit and scope of the invention as definedin the following claims.

What is claimed is:
 1. A method for treating an aqueous slurrycontaining sodium sesquisulfate crystals to recover sodium sulfatetherefrom which comprises:conducting the slurry through an elongategenerally upright mixing zone in a treatment vessel and mixing theslurry in the mixing zone to promote dissolution of the crystals andformation of sodium sulfate solids; conducting the slurry from themixing zone to a separate clarifying zone so that sodium sulfate solidsare carried in the slurry from the mixing zone toward the clarifyingzone and sesquisulfate crystals within the slurry descend from themixing zone toward the clarifying zone; collecting undissolvedsesquisulfate crystals descending from the mixing zone in a reservoir ofa collector located beneath the mixing zone so as to intercept andcollect sesquisulfate crystals descending from the mixing zone towardthe clarifying zone, preventing the collected sesquisulfate crystalsfrom entering into the clarifying zone; bubbling gas through thecollected sesquisulfate crystals to promote dissolution of the crystalsand to provide a flow of gas bubbles ascending through the mixing zoneto promote a turbulent flow regime therein and to assist in mixing ofthe slurry in the mixing zone; collecting sodium sulfate solidsdescending from the slurry in the clarifying zone; conducting thecollected sodium sulfate solids out of the vessel from the clarifyingzone along with entrained liquid consisting principally of sulfuric acidand water; and conducting liquid from the clarifying zone out of thevessel along with a small amount of entrained solids consistingprincipally of sodium sulfate.
 2. The method of claim 1 furthercomprising adding water to the mixing zone to promote dissolution of thesodium sesquisulfate crystals therein.
 3. The method of claim 2 whereinthe ratio of water added to the amount of the sesquisulfate slurry iswithin the range of from about 0.6 to about 1 liter of water perkilogram of sesquisulfate slurry.
 4. The method of claim 2 wherein theratio of water added to sesquisulfate provided to the mixing zone iswithin the range of from about 0.7 to about 0.9 liter of water perkilogram of sesquisulfate slurry.
 5. The method of claim 2 wherein thewater has a temperature within the range of from about 20° to about 100°C.